\startbuffer[DemoPage]
    \hfill
    \vfill    
    \midaligned{\bold{\pagenumber}}
    \vfill
    \hfill
\stopbuffer


\setupbodyfont[ss,120pt]
\setuplayout [margin=0pt, width=fit]
\setuppagenumbering [alternative=doublesided,location=]
\showframe
\setuplayout [location=middle,marking=on]


\setuppapersize[A4][A3,landscape] 
\setuparranging [2UP]

\starttext
    \dorecurse{8}{\getbuffer[DemoPage]\page}
\stoptext

